INTRODUCTORY 3 



is a constant partition-coefficient of growth-intensity between 

 organ and body. It was next found that in many organs, 

 especially those growing at markedly different rates from the 

 body as a whole, growth-intensity was not distributed uni- 

 formly, but in a more or less regular pattern. This led on 

 to the notion, already suggested on different grounds by 

 D'Arcy Thompson, that the growth-intensity of the body as 

 a whole (or, if you prefer it, the relative growth-rates of its 

 various parts) is distributed according to an orderly system 

 of ' growth-gradients '. These conclusions will be discussed 

 in Chapters III to V. 



The physiological mechanism underlying these general rules 

 still remains very obscure, in the absence of experiment 

 specifically directed to the point : but there are some inter- 

 esting hints and possibilities, and these will be discussed in 

 Chapter VI. 



Finally, the facts derived from the study of relative growth 

 have a number of important bearings upon other branches of 

 biology ; and the concluding chapter will be devoted to these. 

 I hope to convince the systematist that by a knowledge of 

 the laws of relative growth we are put in possession of new 

 criteria bearing on the validity of species, sub-species, and 

 ' forms ' ; the nature of certain dwarf forms ; and the import- 

 ance (or the reverse) of size-differences in general for system- 

 atics. In regard to that special branch of systematics usually 

 called physical anthropology, it will be found that these laws 

 have a bearing on the important question as to whether true 

 evolutionary change has taken place in civilized populations 

 during historical time. As regards evolution, it will be found 

 that the subject throws light upon the question of adaptation, 

 on the general theory of orthogenesis, and on the selection 

 problem. Furthermore, the existence of growth-gradients, as 

 D'Arcy Thompson has already pointed out, makes it much 

 easier for us to understand how certain types of evolutionary 

 transformation can have been brought about, since a single 

 genetic change affecting a growth-gradient will automatically 

 express itself in a changed relation in the size of a large 

 number of organs or regions. 



Then comparative physiologists will find it necessary to 

 know precisely how to discount the effects of differences in 

 total absolute size when they wish to estimate the compara- 

 tive development of an organ in a series of related species 

 or groups ; and will further find interesting hints as to the 



